This invention relates to apparatus for application and contouring mastic and more particularly concerns an improved adjustable blade assembly.
In the construction of drywall partitions in various types of buildings, the joint between adjacent panels of wallboard is covered by a strip of tape and a layer of mastic is applied over the tape to provide smooth surface continuity across the edges. The layer of mastic is of a slightly crowned configuration, being somewhat thicker at the center of the tape, and tapering toward feather edges where it blends with the surface of the adjacent drywall panel. The mastic commonly is applied to a taped drywall joint by running a mastic applicator vertically downwardly along the vertical joint and over the tape. Often, a first layer of mastic is applied in a lesser width, such as a width provided by a seven inch applicator and then a second layer is provided using a wider applicator tool.
The mastic applicator tool is supported by a handle which not only carries the weight of the tool and its mastic content but which also presses the mastic from the tool so that the dispensed mastic may be smoothed and contoured by a blade carried at the tool trailing edge. Thus, pressure exerted upon the handle tends to force mastic from the tool and also presses the tool and thus its contouring blade toward the wall to control thickness of applied mastic.
Mastic applying tools of this type are shown in a number of United States patents including the following: U.S. Pat. Nos. 2,571,096, 2,666,323, 2,711,098, 2,824,442, 2,889,699, 2,984,857, and 3,343,202. In these patents the contouring blade is fixedly mounted and adjusted by various means which enable the blade to assume one of a number of cured configurations as may be desired by the operator. Such contouring blades are difficult to adjust. It is often necessary, in order to obtain the proper shape of the resulting contoured mastic, for the operator to both apply an excessively great pressure on the handle and at the same time to move the tool at an undesirably rapid rate. Further, these tools have only a limited number of adjustments and generally attain adjustment by means of exerting pressure upon a flat spring which in turn resiliently presses against members affixed to portions of the blade. These arrangements employ a spring to perform an adjustment of only a single direction, that is, the adjustment can only cause the concavely curved blade to approach a straight condition or an outwardly bowed position. To increas the blade curvature, manual pressure is exerted on the blade as by passing the edge of the blade back and forth across the edge of a door jamb, for example. Thus, the blades are difficult to adjust, can be readily adjusted by the adjusting mechanism only in a single direction and, due at least in part to variable and changing spring characteristics, precision blade adjustment is not available. Prior art blade assemblies, furthermore, have a limited number of adjustment positions and the adjustment mechanism cannot be employed to dislodge or loosen solidifed mastic that may inadvertently remain on the tool after use.
Accordingly, it is an object of the present invention to provide a mastic applying and contouring tool that eliminates or minimizes above-mentioned problems.